1. Field of the Invention
The present invention relates to an electrophoretic device, method of driving electrophoretic device, and electronic apparatus. In particular, the invention relates to an electrophoretic device that has an electrophoretic dispersion liquid including a liquid dispersion medium and electrophoretic particles, to method of driving electrophoretic device, and to electronic apparatus comprising the electrophoretic device which uses the driving method.
Priority is claimed on Japanese Patent Application No. 2005-60532, filed Mar. 4, 2005, the content of which is incorporated herein by reference.
2. Description of Related Art
In relation to an electrophoretic device which has an electrophoretic dispersion liquid including a liquid dispersion medium and electrophoretic particles, there is heretofore known an electrophoretic display device that utilizes the fact that when an electric field is applied to the electrophoretic dispersion liquid, a distribution of the electrophoretic particles is changed and an optical characteristic of the electrophoretic dispersion liquid changes (for example, refer to Japanese Examined Patent Application, Second Publication No. S50-15115). Since such an electrophoretic device does not require a backlight, it can contribute to reducing the cost, and making the display device thinner. Further, the electrophoretic display device has a memory property of the display in addition to a wide angle of visibility and a high contrast. Therefore, it is drawing attention as the next generation display device.
Moreover, there has been proposed a method wherein the electrophoretic dispersion liquid is encapsulated in a microcapsule in an electrophoretic display device (for example, refer to Japanese Unexamined Patent Application, First Publication No. H01-86116). There are advantages by encapsulating the electrophoretic dispersion liquid in a microcapsule in that spilling of the electrophoretic dispersion liquid during the manufacturing process of the electrophoretic display device can be avoided, and precipitation and aggregation of the electrophoretic particles can be reduced.
Furthermore, there is known an electrophoretic display device which is a combination of such an electrophoretic display device and an active matrix device wherein an electric field is applied to the electrophoretic dispersion liquid by operating the active matrix device so that a distribution of the electrophoretic particles is changed (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2000-35775).
A structure of a conventional electrophoretic display device is shown in FIG. 12. FIG. 12A is a plan view of the electrophoretic display device, and FIG. 12B is a sectional view of a pixel portion in the electrophoretic display device.
As shown in FIG. 12A, an electrophoretic display device 1 has a plurality of data signal lines 9, a plurality of scanning signal lines 3 that intersect the data signal lines, switching elements 6 such as transistors that are arranged at intersections of the data signal lines 9 and the scanning signal lines 3, a data signal operating circuit 4, a scanning signal operating circuit 5, and pixel electrodes 7.
Here, the pixel electrodes 7 can be subjected to an electrical influence by appropriately providing data signals to the data signal lines 9 and scanning signals to the scanning signal lines 3, and then controlling the ON/OFF switching of the switching element 6. For example, when a scanning signal which selects only one of a plurality of the scanning signal lines, is provided while some data signal is being provided to the data signal line, the switching element 6 that is connected to the selected scanning signal line turns ON, and then the data signal line 9 and the pixel electrode 7 are essentially conducted. That is, at this time, a signal (voltage) supplied to the data signal line 9 is supplied to the pixel electrode 7 through the switching element 6 that is ON. In contrast, a switching element that is connected to the unselected scanning signal line remains OFF, and the data signal line and the pixel electrode are essentially non-conducted.
In this manner, since the electrophoretic display device can selectively turn ON/OFF only the transistor that is connected to a desired scanning signal line, a cross talk problem hardly occurs and it is possible to speed up the circuit operation.
As shown in the sectional view of FIG. 12B, in a general example of a conventional electrophoretic display device, the pixel electrode 7 and a common electrode 8 are provided to oppose each other with a predetermined space therebetween (normally from several μm to several tens of μm). In the space formed between the electrodes, an electrophoretic dispersion liquid 10 that includes a liquid dispersion medium 11 and electrophoretic particles 12 is enclosed. Here, for the sake of simplification, the data signal line and the scanning signal line are omitted in FIG. 12B.
With such a structure, when the above-mentioned operation is conducted and a desired data signal (voltage) is supplied to the pixel electrode 7 while maintaining the common electrode 8 at a predetermined voltage, the electrophoretic particles 12 migrate according to a voltage potential difference (electric field) generated between the common electrode and the pixel electrode, and the spatial distribution is changed. For example, when the electrophoretic particles 12 are positively charged, if the earth potential (0V) is supplied to the common electrode 8 and a negative voltage is supplied to the pixel electrode 7, then the electrophoretic particles 12 are attracted onto the pixel electrode. Conversely, if a positive voltage is supplied to the pixel electrode 7, the electrophoretic particles 12 are attracted onto the surface of the common electrode that is opposed to the pixel electrode. The movement goes the other way around when the electrophoretic particles 12 are negatively charged. Based on such a principal, a desired image can be obtained by appropriately controlling the data signal (voltage) provided to each pixel.
Moreover, as a method for realizing the gradation expression in a conventional electrophoretic display device, there is known a method, referred to as area gradation, wherein a plurality of minute pixel pieces are collected to constitute one pixel and the gradation display of overall pixels is obtained by ON/OFF combination of the respective minute pixel pieces (for example, refer to Japanese Unexamined Patent Application, First Publication No. S50-51695). In the area gradation, each pixel displays either one of; a first optical characteristic state (for example, a state where all electrophoretic particles are deposited on the pixel electrode in FIG. 12B), and a second optical characteristic state (similarly, a state where all electrophoretic particles are deposited on the surface of the common electrode opposed to the pixel electrode in FIG. 12B). Moreover, regarding a plurality of pixels included in a certain region, by adjusting the proportion of the number of pixels displaying the first optical characteristic state and the number of pixels displaying the second optical characteristic state, the average optical characteristic in the region can display the value between the first optical characteristic and the second optical characteristic. Here, in order to make a pixel display the first optical characteristic state, a first voltage is applied to the pixel. On the other hand, in order to make a pixel display the second optical characteristic state, a second voltage is applied to the pixel. In the above example, a negative voltage becomes the first voltage and a positive voltage becomes the second voltage.
The area gradation is further specifically described. As shown in FIG. 13, a display region 2 comprising four pixel electrodes 7 is taken into consideration. Here, the first optical characteristic state is black and the second optical characteristic state is white. In FIG. 13A, the first voltage is applied to all pixels, therefore displaying the first optical characteristic state (that is, the proportion is 4:0). In FIG. 13B, the first voltage is applied to three pixels and the second voltage is applied to the remaining one pixel. As a result, the three pixels display the first optical characteristic state and the remaining one pixel displays the second optical characteristic state (that is, the proportion is 3:1). The proportion is changed in the order of 2:2, 1:3, and 0:4 as shown in C, D, and E. In such a case, the average optical characteristic for the whole region is clearly the first optical characteristic in FIG. 13A and the second optical characteristic in FIG. 13E. However, in the state therebetween, the average optical characteristic becomes the optical characteristic proportionally distributed between the first optical characteristic and the second optical characteristic corresponding to the proportion of the pixel number in the first optical characteristic state and the second optical characteristic state.
For example, the reflectance is considered as the optical characteristic, and it is assumed that the reflectance of the black pixel is Rb and the reflectance of the white pixel is Rw. At this time, the average reflectance in the overall region in FIG. 13A to FIG. 13E becomes as follows respectively.
FIG. 13A: (4Rb+0Rw)/4=Rb
FIG. 13B: (3Rb+Rw)/4
FIG. 13C: (2Rb+2Rw)/4=(Rb+Rw)/2
FIG. 13D: (Rb+3Rw)/4
FIG. 13E: (0Rb+4Rw)/4=Rw
That is, corresponding to the proportion of the white and black pixel number, the reflectance proportionally distributed between Rb and Rw can be expressed.
In such an area gradation, since the gradation is determined by the digital value as the proportion of the pixel number, it is hardly affected by the characteristic difference by each pixel. Furthermore, since it can be controlled by a digital circuit without requiring an analog circuit such as a digital/analogue converter, it is effective in simplifying the control circuit and improving the reliability. However, conversely, since the displayed gradation becomes the average value in a certain region as described above, there is a problem in that, if the pixel size is too large, averaging is not performed by the naked eye and the image appearance is worsened. However, regarding this point, since miniaturization of the pixel size is well advanced due to high quality thin-film circuits, for example represented by a low temperature polysilicon thin-film transistor, it is not considered to become a big problem in the future.
However, there are the following problems in the conventional techniques.
In an electrophoretic display device, electrophoretic particles are deposited ideally on the pixel electrode or the surface of the common electrode opposed to the pixel electrode. However, actually in some cases, electrophoretic particles overflow the ideally deposited region due to the leakage of the electric field passing through the electrophoretic dispersion liquid.
The case is described with reference to the drawings. For example, in the electrophoretic display device having the structure shown in FIG. 12B, as described above, when the electrophoretic particles 12 are positively charged, if the earth potential (0V) is supplied to the common electrode 8 and a positive voltage is supplied to the pixel electrode 7, then the electrophoretic particles 12 are attracted onto the surface of the common electrode opposed to the pixel electrode. At this time, ideally as shown in FIG. 14A, the electrophoretic particles 12 are deposited only in a region on the common electrode opposed to the pixel electrode. However, actually in some cases, since the electric field from the pixel electrode to the common electrode leaks horizontally to some degree, the particles overflow from the ideal region and are deposited as in FIG. 14B, or they are deposited inside of the ideal region as in FIG. 14C. In such a case, the pixel size in appearance viewed from the common electrode side becomes larger in FIG. 14B, and smaller in FIG. 14C, than the actual pixel electrode size. Furthermore, if the structure is such that a plurality of pixel electrodes are arranged in matrix form, the manner of leaking differs according to the state of voltage applied to the adjacent pixel electrode. Consequently, in the actual area gradation, even if the first voltage or the second voltage is appropriately applied to respective pixels in order to obtain the desired proportion of the pixel number of the first optical characteristic state, and the pixel number of the second optical characteristic state, the pixel area ratio in appearance becomes different, causing a problem of inability to obtain the desired optical characteristic.